Carbon monoxide removal of discolorants from chromate oxidation products



July 30, 1968 COLEMAN, JR 3,395,177

CARBON MONOXIDE REMOVAL OF DISCOLORANTS FROM CHROMATE OXIDATION PRODUCTS Filed May 6, 1965 2,6 DIMETHYL- NAPHTHALENE IN VENTOR JOHN R. COLEMAN JR,

ATTORNEY CARBON MONOXIDE REMOVAL OF DIS- COLURANTS FRUM CHROMATE OXI- DATIGN PRODUQTS John R. Coleman, Jr., Littleton, (1010., assignor to Marathon Oil Company, Findlay, Ohio, a corporation of Ohio Fiied May 6, 1965, Ser. No. 453,668 9 Claims. (Cl. 260-524) ABSTRACT OF THE DISCLOSURE The present invention comprises a process for the preparation of chromate oxidation products by the oxidation of alkyl aromatic hydrocarbons in the presence of CO and soluble inorganic dichromate salts, the steps comprising in combination adjusting the pH of the prod uct mixture obtained from the oxidation as necessary to obtain a pH in the range of from about 8 to about 12 and adjusting the temperature to from about 150 to about 350 F., thereafter adding to the product mixture from the oxidation reactor about 1 to 3 moles per mole of chromium VI in the product mixture of carbon monoxide and removing the precipitate.

The present invention relates to processes for the production of chromate oxidation products free from discoloring heavy metals, and in particular, the present invention relates to methods for the treatment of product mixtures containing the metal salts of carboxylic acids with carbon monoxide to remove heavy metals prior to the conversion of the salts to the corresponding acids.

The oxidation products of alkyl aromatic hydrocarbons, and particularly those obtained by chromate oxidation in the presence of CO are useful for the production of polyesters, alkyd resins, and various other polymeric materials. The color of such polymeric materials is important to their commercial value and the products of conventional chromate oxidations have often tended to gradually discolor.

The present invention embodies the discovery that this discoloration is primarily caused by the presence of heavy metals and the invention provides a method for the removal of these heavy metals after the oxidation is complete. The metal removal is effected by the present invention without the necessity of expensive processing equipment or a large number of purification steps. The sodium sulfate by-product normally produced in such chromate oxidations is not contaminated by the processes of the present invention and is, therefore, not decreased in commercial value. In addition, no additional ions are present in the product mixture after the process is complete.

The unreacted chromium or other heavy metal compounds obtained from the reactor effluent by the process of the present invention are easily recycled to a regeneration system thereby eliminating the losses of these expensive oxidants commonly encountered in conventional processes.

FIGURE 1 is a schematic diagram of the preferred embodiment of the present invention.

In FIGURE 1 about 250 pounds of 2,6-dimethylnaphthalene is fed to an oxidation reactor 11 together with about 220 pounds of CO (sufiicient to pressurize the reactor to a total pressure of about 1,500 p.s.i.g. at reaction temperature) and about 1,022 pounds of a mixture containing about 65% Na Cr O and being dissolved in about 2,000 pounds of water. This mixture is f States Patent agitated and the reaction is permitted to continue for about two hours after which the mixture is cooled and the unreacted dimethylnaphthalene is separated from the product mixture by decanting. The temperature is then adjusted to from to about 350 F., preferably from to 250 F. The product mixture, still remaining in the oxidation reactor, is then treated with preferably from about 1 to 3 moles per mole of carbon monoxide per mole of chromium VI (suflicient to pressurize the reactor to about 100 p.s.i.g.). Immediately, a substantial quantity of chromic oxide precipitates, and the orange color of the liquid phase changes abruptly to a deep green. This color change is indicative of a conversion of unreacted soluble chromium VI to dispersed solid chromium III. The conversion is nearly quantitative, and virtually all chromium VI is removed from the solution. The pH of the product mixture is adjusted as necessary to be at least about 8 and preferably from 8 to 12 at this point. The product mixture is filtered using a conventional type of plate and frame filter 12, with No. 50 or similar filter paper between the plates. The filtrate thus produced is essentially colorless. The solution is then acidified in acidification reactor 13 with sulfuric acid to provide a pH of about 2, thus precipitating 2,6-dicarboxynaphthalene. The 2,6-dicarboxynaphthalene thus produced is separated out in filter 14 and is found to be virtually pure White in color indicating that it is free from substantial quantities of either chromium VI or chromium III. Analysis confirms the substantial absence of chromium and also indicates that no substantial quantities of iron are contained in the product. Because the process of the present invention removes the iron together with the chromium, the glass-lined reaction vessels commonly used for such preparations are not necessary, and less expensive steel vessels may be employed in most instances.

In particularly preferred embodiments, the solid material removed by the first filtration, which is composed mainly of Na CO NaHCO and Cr O is sent to a regenerator (shown in FIGURE 1 as element 15) where it is mixed preferably with approximately twice stoichiometric quantities of oxygen, preferably in the form of air and with an alkali base metal, preferably NaOH, NaHCO or Na CO in order to generate Na CrO The Na CrO thus produced is returned to the oxidation reactor for use in later oxidations. It is preferable to use the same alkali metal cation throughout the process.

It should be noted that the above example employs only an oxidation reactor, filter press (in batch operations, filters 12 and 14 may be combined) and storage tank, all of which would normally be employed in the conventional preparation of 2,6-dicarboxynaphthalene. Thus, in preferred embodiments, the present invention requires the use of no additional equipment and instead, permits the use of reactors less expensive than those conventionally employed. Since the second acidification with sulfuric acid is required in conventional preparations, the additional steps required for the practice of the present invention are merely the addition of the carbon monoxide and the first pH adjustment which is not necessary in certain circumstances as discussed hereinafter.

It should be understood that while it is preferred, sodium is intended to be merely exemplary of the cations which may be employed and any of the corresponding alkali metal salts having suitable solubility and reactivity and even the corresponding salts of other suitable cations, e.g., ammonium, may :be employed in the process.

The filtrate from the second filtration (in which the 2,6-dicarboxynaphthalenes are removed as precipitate) is primarily a solution of Na SO in the above preferred embodiment. This is the by-product normally obtained in conventional sodium chromate oxidation reactions, when using sulfuric acid for precipitating the organic acid. The

process of the present invention in no way contaminates or otherwise reduces the value of the by-product of the sulfate. When it is not necessary to recover high purity Na SO any mineral acid such as HCl or H PO can be used in place of sulfuric acid to precipitate the 2,6-dicarboxynaphthalene.

In place of the above described final acidification with sulfuric or other mineral acids, the processes of copending application of the same assignee, Serial No. 420,502 filed December 23, 1964, by J. W. Patton and M. 0. Son, Jr., may be used. Also, the purification process described in copending application of the same assignee, Serial No. 420,503 filed December 23, 1964, by J. W. Patton and M. 0. Son, Jr., may be employed with the present invention.

The CI2O3 which precipitates in the course of oxidizing the aromatic hydrocarbon in reactor 11 in the above embodiment may optionally be removed by conventional means prior to the addition of carbon monoxide. The optimum pH will vary somewhat according to the raw materials being oxidized. pH adjustment is usually not necessary when using NaHSO or Na SO Where necessary, the pH adjustment can be accomplished by the addition of any common alkali having suitable solubility and not causing undesirable side reactions. In most cases, the hydroxides or carbonates of sodium or potassium will be preferred. In order to prevent the addition of extraneous cations, it is especially desirable that the alkali used in pH adjustment have the same cation as the chromate used in the oxidation step.

The acidification which follows the first filtration is utilized primarily to convert the products of the oxidation, generally metal salts of carboxylic acids, to the corresponding carboxylic acids. This conversion may also be accomplished by ion exchange or other methods as taught by the copending applications mentioned above. Where acid is utilized, in general the pH will preferably be below about 2 and most preferably from about 1 to 1.9. However, as in the first pH adjustment, optimum pH range will vary somewhat with the particular raw materials being oxidized and may be readily determined by trial runs.

In place of the filtrations described above, centrifugations or other conventional separatory processes may be employed, and filtering as used herein includes all such methods. The entire process may, of course, be conducted on either a batch or continuous basis.

The invention may be employed advantageously with a wide variety of alkyl aromatics including among others: monoalkylbenzenes such as toluene, ethylbenzene, propylbenzene; and butylbenzene; dialkyl benzenes such as dimethyl'benzenes, diethylbenzenes, dipropylbenzenes and dibutylbenzenes; and other polyalkylbenzenes such as trimethylbenzenes, triethylbenzenes and tripropylbenzenes; monoalkylnaphthalenes such as methyl-, ethyl-, and propylnaphthalenes; dialkylnaphthalenes such as dimethyl-, diethyl-, and dipropylnaphthalenes; polyalkylnaphthalenes such as trimethylnaphthalenes, tetramethylnaphthalenes, and dimethyldiethylnaphthalenes; and the corresponding tetralin derivatives, alkylphenanthrenes, alkylanthracenes, etc. Especially preferred are the mono and dialkylbenzenes and naphthalenes wherein the alkyl groups contain from 1 to about 6 carbon atoms, and of these, those having methyl groups are most preferred.

EXAMPLE I Preparation of 2,6-dicarbxynaphthalene free from discoloring heavy metal impurities Utilizing an equipment system similar to that described in FIGURE 1, there is fed to a 500 gallon pressure type agitated oxidation mild steel reactor 257 pounds of 2,6- dimethylnaphthalene (a excess over stoichiometric quantity), roughly 220 pounds of CO (an amount necessary to maintain the below mentioned reactor pressure), and an aqueous Na Cr O /Na CrO solution consisting of 671 pounds of sodium dichromate (Na Cr O 2H O), 351 pounds of sodium chromate (Na CrO -4H O), and 2,050

pounds of water. The oxidation reactor operates at about 1,500 p.s.i.g., maintained by the CO feed and at about 260 C. This pressure and temperature are maintained in the reactor for a reaction time of about 4 hours. After recovery of the unreacted alkylaromatics (amounting to about 10% of the total 2,6-dimethylnaphthalene added) for later recycle to the reactor, the remainder of the reaction mixture containing solid Cr O NaHCO and Na CO and a solution of the above carbonates and the sodium salt of 2,6-dicarboxynaphthalene is treated with carbon monoxide by pressurizing the reactor with stirring to a pressure of p.s.i.g. at about 200 F. Chromic oxide precipitates, changing the orange liquid phase product to a deep green. The product mixture is then filtered through a plate and frame filter using No. 50 or similar filter paper and the chromic oxide thus produced and that formed in the oxidation (about 540 pounds total Cr O -1.5H O) is retained for regeneration and reuse. The pH of the filtrate is approximately 8 to 9. The filtrate is then acidified with about 190 pounds of sulfuric acid which causes white 2,6- dicarboxynaphthalene to precipitate. The precipitate is removed by filtering as before, and the filtrate is evaporated to recover about 270 pounds of Na SO which is found to be free of iron or heavy metals. About 305 pounds of dicarboxynaphthalene is recovered and is found to be of excellent whiteness with virtually no heavy metal content.

The Cr O recovered by the first filtration is heated with about 2,500 pounds of NaHCO while under oxygen pressure to regenerate it to a mixture of Na CrO /Na Cr O for use in subsequent oxidations.

In summary, the invention comprises in a process for the preparation of chromate oxidation products by the oxidation of alkyl aromatic hydrocarbons in the presence of CO and soluble inorganic dichromate salts, the steps comprising in combination adjusting the pH of the product mixture obtained from the oxidation to from about 8 to about 12 and adjusting the temperature to from about to about 350 -F., thereafter adding to the product mixture from the oxidation reactor about 1 to 3 moles per mole of chromium VI in the product mixture of carbon monoxide and removing the precipitate.

The foregoing example and discussions are merely illustrative and the invention is to be understood to include all of the variations and modifications which are apparent to those skilled in the art.

What is claimed is:

1. In a process for the preparation of chromate oxidation products by the oxidation of alkyl aromatic hydrocarbons in the presence of CO and soluble inorganic dichromate salts, the steps comprising in combination adjusting the pH of the product mixture obtained from the oxidation as necessary to obtain a pH in the range of from about 8 to about 12 and adjusting the temperature to from about 150 to about 350 F., thereafter adding to the prod uct mixture from the oxidation reactor about 1 to 3 moles per mole of chromium VI in the product mixture of carbon monoxide and removing the precipitate.

2. The process of claim 1 wherein the precipitate removed is treated with an oxygen-containing gas and an alkali metal base in order to produce a soluble alkali metal dichromate for recycle to the oxidation process,

3. The process of claim 2 wherein the alkali metal is in all cases sodium.

4. A process for the preparation of carboxy aromatic acids substantially free from heavy metal contaminants comprising in combination the steps of oxidizing alkyl aromatic hydrocarbons in the presence of CO and soluble inorganic dichromate salts, thereafter adjusting the pH as necessary to obtain a pH in the range of from about 8 to about 12, and adjusting the temperature to from about 150 to about 350 F., thereafter adding to the product mixture from the oxidation reaction about 1 to 3 moles per mole of chromium VI in the product mixture of carbon monoxide, filtering and adjusting the pH of the filtrate to below about 2 and thereafter refiltering the filtrate at a temperature of from about 30 to about 250 F. to recover the acid precipitate produced.

5. The process of claim 4 wherein the alkyl hydrocarbons are selected from the group consisting of monoalkylbenzenes, dialkylbenzenes, monoalkylnaphthalenes, dialkylnaphthalenes, and wherein the alkyl hydrocarbon groups contain from 1 to about 6 carbon atoms.

6. The process of claim 4 wherein the alkyl hydrocarbons comprise compotmds selected from the group consisting of toluene, xylenes, monomethylnaphthalenes, and dimethylnaphthalenes.

7. A process for the preparation of alkali metal salts of aromatic acids which salts are substantially free from heavy metal contaminants comprising in combination the steps of oxidizing alkyl aromatic hydrocarbons in the presence of CO and soluble inorganic dichromate salts, thereafter adjusting the pH of the product mixture as necessary to obtain a pH in the range of from about 8 to 12 and the temperature to from 150 to about 350 F., thereafter adding to the product mixture from the oxidation reaction about 1 to 3 moles per mole of chromium VI in the product mixture of carbon monoxide, thereafter filtering and 6 recovering said alkali metal salts of aromatic acids from the resulting filtrate.

8. The process of claim 7 wherein the alkyl hydrocarbons are selected from the group consisting of monoalkylbenzenes, dialkylbenzenes, monoalkylnaphthale-nes, dialkylnaphthalenes, and wherein the alkyl hydrocarbon groups contain from 1 to about 6 carbon atoms.

9. The process of claim 7 wherein the alkyl hydrocarbons comprise compounds selected from the group consisting of toluene, xylenes, monomethylnap-hthalenes, and dimethylnaphthalenes.

References Cited UNITED STATES PATENTS 2,005,774 8/1932 Demant 260-524 2,544,687 4/1947 Kalbach 23-145 3,282,992 7/ 1964 Harris 260-524 1,945,032 8/ 1932 Demant 260-524 HENRY R. JILES, Primary Examiner.

D. STENZEL, Assistant Examiner. 

